1. Field of the Invention
The invention relates to an ion optical system that extracts and forms an ion beam which can be used for ion implantation processes, particularly in the low energy range 100 eV-4 keV. The invention enables a broad energy range of the transported ion beam and also enables the extraction of molecular ions as well as more conventional monomer ion beams using a simple triode extraction structure. Novel features are incorporated into the invention that enable beam formation and variable focusing of ion beams over a very broad range of beam current, ion mass and source brightness, while being compatible with many commercial beam line implantation platforms.
2. Description of the Prior Art
—Ion Implantation Process
The ion implantation process relies on ionizing gaseous or vaporized solid feedstock material in an ion source and extracting either positive or negative ions from the source through an extraction aperture using electric fields. The beam is then mass analyzed, transported and implanted to target semiconductor wafer.
—Ion Source and Extraction
In traditional implanter ion sources, arc discharge or RF excitation is typically used to form a dense plasma, which is a mix of thermal electrons, fast ionizing electrons, and ions. FIG. 1 shows a schematic of a traditional plasma ion source used in implanters. The ion beam is extracted from the source through an opening in the source wall. The extraction aperture shape is traditionally a slot with a width of a few millimeters and height of few tens of millimeters. The ion source and extraction aperture plate are typically at the same potential, but sometimes a voltage is applied between the two. A suppression electrode that is at negative potential is used to form the electric field that pulls the ions out of the source. It also creates a potential barrier for back streaming electrons that are formed downstream through beam impact on surfaces or background gas ionization. A third electrode follows the suppression electrode which is at the ground potential.
Typically the suppressor and the ground electrode are a movable unit in order to change the gap between the extraction aperture plate and the suppression electrode. This is required as the ion beam final energy, which is set by the source potential, is varied and the electric field in the extraction gap has to be adjusted accordingly in order to maintain the same extraction conditions for the ion beam. This relation stems from the fact that the extracted current density depends on the extraction electric field through Child's law:
                              j          =                      1.72            ⁢                                          Q                M                                      ⁢                                                            U                                      3                    /                    2                                                                    d                  2                                            ⁡                              [                                  mA                  ⁢                                      /                                    ⁢                                      cm                    2                                                  ]                                                    ,                            (        1        )            
Where j is the maximum extractable current density of the ion beam, Q and M are the charge state and the mass number of the ion and U [kV] and d [cm] are the applied voltage and gap between the ion source body/extraction aperture plate and the suppression electrode, respectively. Child's law gives the space charge limit for the extractable current density from the ion source.
FIG. 2 shows a schematic of a typical ion implanter extraction system. The ion extraction aperture is either a round aperture or a slot with a chamfer on the downstream side of the aperture. This chamfer angle α varies typically from 35 to 75 degrees, most typically a so-called Pierce angle of 67.5 degrees is used. The thickness of the extraction aperture plate is normally 6 mm or less. The shape of the suppression/extractor electrode often features a protruding lip that can be brought into close proximity to the aperture plate. The schematic of FIG. 2 is represents typical dispersive (horizontal) plane optics. In the non-dispersive (vertical) plane the extraction slot is usually much taller than the dispersive plane width of the slot, making the dispersive and non-dispersive plane optics separable in their mathematical representation. To effect non-dispersive plane focusing of the beam, the extraction aperture plate and the suppression and ground lips are typically curved. The radius of curvature (along the long axis) is optimized to match the beam acceptance of the analyzer magnet and subsequent beam line.
FIG. 3 shows a schematic of typical non-dispersive plane electrode shapes. The beam analyzer magnet focuses the beam in the dispersive plane. The beam width at the exit of the analyzer dipole magnet is related to the width of the beam at the entrance of the magnet by equation 2:y2=y1 cos(α1),  (2)
Where y1 and y2 are the beam half-widths at the entrance and exit field boundaries, respectively, and α1 is the magnet sector angle. If the sector angle is smaller than 90 degrees, the beam leaves the magnet converging. At a 90 degree sector angle the beam has a focal point at the magnet exit, and with a sector angle larger than 90 degrees the beam has a focal point inside the magnet and leaves the magnet diverging.
The requirement set for the extraction optics will be the ability to form a beam that has small enough divergence and beam size in the dispersive plane to match the acceptance of the analyzer magnet. In the non-dispersive plane, the beam focusing can be accomplished by the curvature of the electrodes, but additionally the analyzer magnet can have some focusing properties either through pole rotation or pole face indexing.
—Space Charge Forces
It can be problematic to achieve a desired beam focusing in the non-dispersive plane if the space charge of the beam is varying significantly between different operation modes of the extraction system. The space charge of the beam depends on beam energy and current. The transverse space charge force FSPC,SLIT acting on the envelope of the ion beam can be written for a slit beam in a following form:
                              F                      SPF            ,            SLIT                          =                  eJ                      2            ⁢                                                  ⁢                          ɛ              0                        ⁢            v                                              (        3        )            
In equation (3), e is the elementary charge, J is the beam current per unit length of the slot, ε0 is the permittivity of free space and v is the directed velocity of the particle along the beam direction. For round beam the same equation can be written in form:
                              F                      SPC            ,            ROUND                          =                  qI                      2            ⁢                                                  ⁢            π            ⁢                                                  ⁢                          ɛ              0                        ⁢                          vr              0                                                          (        4        )            where q is the total charge of the ion, I is the beam current and r0 is the beam envelope radius.
The space charge forces described in equations (3) and (4) are transverse forces with respect to the beam direction, which will blow up the beam as it drifts in the beam transport system. This has implications for the extraction of the ions from the ion source. Ideally, the extraction optics should be designed so that the resulting electric fields will compensate the transverse space charge force and form an approximately parallel, or only slightly diverging, beam in the dispersive plane, while focusing or containing the beam envelope in the non-dispersive plane.
In typical ion implanters atomic ion species are used to form the implanted beams of boron, arsine and phosphorus. The extracted current densities can be in the range of a few mA/cm2 and higher. This sets boundary conditions for the design of the extraction optics in the existing implanters. Typically slit extraction is used with slit sizes of a few mm in width (dispersive plane) and 20-40 mm in height (non-dispersive plane). The extraction gap between the aperture plate and the suppression electrode typically varies from a few mm to a few tens of mm when the beam energy is in the range used in implanters, which is from a few hundred eV to 80 keV.